The field of the invention is gyromagnetic resonance spectroscopy, and particularly, local coils which are used to receive the signals produced by gyromagnetic resonance system.
Gyromagnetic resonance spectroscopy is conducted to study nuclei that have magnetic moments and electrons which are in a paramagnetic state. The former is referred to in the art as Nuclear Magnetic Resonance (NMR), and the latter is referred to as Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR). There are other forms of gyromagnetic spectroscopy that are practiced less frequently, but are also included in the field of this invention.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z), the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z) is produced in the direction of the polarizing field, but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the excitaton signal B.sub.1 is terminated. When the excitation signal is removed, an oscillating sine wave referred to as an NMR signal is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.o, is determined by the magnitude of M.sub.1.
The NMR systems which implement these techniques are constructed in a variety of sizes. Small, specially designed machines are employed to examine laboratory animals or to examine specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof. Whole body scanners may employ an excitation coil for producing the excitation field and a separate receiver coil for receiving the NMR signal. The excitation coil produces a highly uniform, or homogeneous, excitation field throughout the entire area of interest, whereas the receiver coil is placed near the immediate area of interest to receive the NMR signal. The receiver coil is sharply tuned to the Larmor frequency of the nuclei of interest.
Recently, a novel resonator structure referred to in the art as a "loop-gap" resonator has been applied to the field of gyromagnetic resonance spectroscopy. As indicated in the U.S. Pat. Nos. 4,435,680; 4,446,429; 4,480,239 and 4,504,788, the loop-gap resonator may take a wide variety of shapes. In all cases, however, a lumped circuit resonator is formed in which a conductive loop is the inductive element and one or more gaps are formed in this loop to form a capacitive element. While the loop-gap resonator has many desirable characteristics normally associated with lumped circuit resonators, it also has some characteristics normally associated with cavity resonators. Most notable of these is the much higher quality factor, or "Q", of the loop-gap resonator over the traditional lumped circuit resonators. When applied to NMR scanners, this higher Q provides a higher signal-to-noise ratio which translates into higher resolution images and shorter scan times.
To enhance the signal-to-noise ratio (SNR) of the receiver coil, it is common practice to reduce its physical size and place it adjacent the immediate region of interest. Such receiver coils are referred to as "surface coils", or "local coils", because their reception field is limited in size to the local area. For example, in U.S. Pat. No. 4,724,389, local coils specifically designed for producing images of the head and neck regions of a human subject are described. In general, the more nearly the reception field of the local coil matches the field of interest, the better the resulting NMR signal.